1. Field of the Invention
The present invention relates to a sensor for contactless determination of an angle of rotation rotatable structural element.
2. Description of the Related Art
Sensors are classified into two basic types, i.e., contact-type ones and contactless-type ones. A typical one of the contact-type is constructed of a resistor element and a rotating brush kept in contact with the resistor element. The contact-type sensors are simple in construction and low in manufacturing cost. However, any of the contact-type sensors is at a disadvantage in being shortened in service life due to wear on the contact areas of its components. On the other hand, the contactless-type sensors are free from the above disadvantage. A typical one of the contactless-type sensors is constructed of: a stator provided with a pair of coils and, a rotor provided with at least one magnetic-substance portion which is so arranged to vary the distance between the magnetic-substance portion and each of the coils when the rotor rotates to vary its angle of rotation relative to the stator. In the typical one of the contact-less type sensors, the angle of rotation, i.e., angular displacement of the rotor, is determined through detection of variations in voltage in a circuit caused by variations inductance in the pair of the coils forming the circuit.
As for the contactless-type sensors, Japanese Patent Laid-Open No. Sho 59-174702 discloses a concrete example of the sensors, in which a stator is constructed of a cylindrical yoke member and a pair of coils oppositely disposed from each other in a diametrical direction on a line perpendicular to a central axis of the cylindrical yoke member. In the above example, a rotor of the sensor is constructed of a semicylindrical magnetic-substance element which is rotatable on the central axis of the cylindrical yoke member. In operation, the rotor assuming the semicylindrical shape is detected in angular displacement by the pair of coils, in which the angular displacement of the rotor is converted into variations in conductances of the coils, i.e., variations in voltage in a circuit comprising the coils, which permits the example of the sensor to determine the angular displacement of the rotor through detection of the variations in voltage.
This example, i.e., the conventional sensor for contactless determination of the angular displacement of the rotor, has the disadvantage that the stator is too complex in manufacturing. In other words, in the stator of the conventional sensor, the pair of coils are disposed on an inner peripheral surface of the cylindrical yoke member so as to be oppositely disposed from each other. In manufacturing such coils, first each of the coils is formed; and then the formed coils are disposed in predetermined positions on the inner peripheral surface of the yoke member through assembly operations. In this case, since the conventional sensor is directly influenced in its determination accuracy by variations in the installation positions of the coils. It is necessary for the conventional sensor to perform the positioning operations or assembly operations of the coils with a very high accuracy, which impairs the conventional sensor in productivity.